Because machines are thrown away these days around 4 years after then are purchased. It is like arguing why should I throw away 10 day old bread thats only slightly moldy and dried out to a hungry guy who has a supply of 2 day old bread for free.
On an individual basis you can probably find somebody that would want a 10 year old Pentium, but for most organizations they would rather deal with 4 year old Dells that all can run XP and have a legit XP sticker/serial. If something does break people would rather be able to get spare parts from anywhere and not having to use ebay to hunt for obsolete stuff.